The present invention generally relates to a method of handling asymmetrical transmissions, specifically information transported by means of transmission control protocol (TCP) via asymmetrical error prone channels.
A demand for mobile high capacity data transfers have put high capacity, but unfortunately asymmetrical, wireless broadcasting transmission channels in focus. The majority of all information transfers today are made with the help of Transmission Control Protocol (TCP). Unfortunately this protocol is not designed for neither asymmetrical transmission or wireless error prone transmissions, such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting Terrestial (DVB-T), and Wavelan. Due to this there has come about many different suggestions such as replacing TCP with other modern protocols and modifying TCP to different varying degrees. For information/data transfer via wireless transfer channels special measures have to be taken to be able to manage a low as possible transmitting power. The measures can, for example, be the use of error correcting codes or special retransmission protocols.
To be able to handle bit errors with TCP, a Selective Acknowledgement (SACK) option to Acknowledgements (ACKS) have been developed. By means of the SACK option the TCP transmitter can get to know which packets/segments after the first lost segment that have reached their destination. A SACK option can normally give information of up to three contiguous blocks of segments that are correctly received and also three contiguous blocks of missing or erroneous segments in the information transmission. The use of SACK options presupposes that there is no significant asymmetry in the network so that as many SACIs as information segments transmitted downlink, can be transmitted uplink.
One way of dealing with asymmetry in an extremely asymmetrical network is called ACK-filtering and works on the principles that many of the ACKs that are transmitted back to the transmitter are redundant and do not have to be transmitted. If there is a limitation in the return channel there will emerge a queue and it is in this queue that the ACK-filtering operates.
A combination of these two methods will create problems. SACK options are based on the assumption that there is no limitation in the return channel, and they are thus designed accordingly, and if an ACK-filtering is operated on a SACK queue, i.e. a queue of ACKs with SACK options, then some SACK blocks will be completely removed and then the transmitter will not find out that these blocks have been received. This will lead to the transmitter erroneously retransmitting the segments in these blocks that have already been received. This will result in the downlink transmission channel suffering when bandwidth is used for unnecessary retransmissions.
The problems associated with asymmetrical networks in conjunction with TCP needs to be solved satisfactorily. TCP can handle an asymmetry factor of 50 times the downlink to the uplink in most common implementations. Future networks can have an asymmetry factor of around a 1000 times. There seems to be room for improvement.
An object of the invention is to define a method for efficiently transferring information fn an asymmetrical network.
Another object of the invention is to define a method of efficiently using TCP with ACKs comprising SACK options in an asymmetrical network.
The aforementioned objects are achieved according to the invention by a method of reducing the number of ACKs with SACK options to be transferred back to a sender. New ACKs with SACK options are compared to previous ACKs with SACK options as to their content, and in dependence of the comparison, i.e. if the previous ACK with a SACK option comprises redundant or invalid information and no additional information in relation to the new ACK with a SACK option, then the previous ACK with the SACK option is removed and not sent back to a sender.
The aforementioned objects are also achieved by a method of, when receiving information in a network that transports information according to the general principles of the Transmission Control Protocol (TCP), reducing the number of selective acknowledgements (SACKs) to be transferred back to a sender. Selective Acknowledgements, SACKs, will generally be used for referring to Acknowledgements (ACKS) with Selective Acknowledgement options. The method comprises a plurality of steps when a new selective acknowledgement has been triggered/generated. In a first step determining if there is a previous selective acknowledgement in a queue to be transferred to the sender, and if there is a previous selective acknowledgment in the queue then further performing the following additional steps. In a first additional step comparing the content of the new selective acknowledgement with the content of the previous selective acknowledgement. Finally in a second additional step removing the previous selective acknowledgement from the queue in dependence on the result of the comparison.
Advantageously in the second additional step of removing the previous selective acknowledgement from the queue, the previous selective acknowledgement is removed from the queue if the previous selective acknowledgement has a different number of blocks than the new selective acknowledgement. Further in the second additional step of removing the previous selective acknowledgement from the queue, the previous selective acknowledgement can and/or also advantageously be removed from the queue if the previous selective acknowledgement has the same number of blocks as the new selective acknowledgement and if only the right edge of the first block is different between the previous selective acknowledgement and the new selective acknowledgement.
In some versions of the invention the method can further advantageously comprise the additional step of sorting the selective acknowledgements in the queue as to which blocks are redundant. Preferably the step of sorting comprises the following four block sorting steps. A first block sorting step of disassembling into blocks the selective acknowledgements that have not been transferred to a sender. A second block sorting step of removing redundant blocks. A third block sorting step of assembling the remaining blocks into block sorted selective acknowledgements. A fourth block sorting step of placing the block sorted selective acknowledgements into the queue. The new selective acknowledgement is preferably not disassembled, but the knowledge of which blocks are comprised in the new selective acknowledgement is used in the second block sorting step for removing redundant blocks. In some versions there are status indicators to keep track of one or both of if and when a queue has been sorted with regard to selective acknowledgements.
The step of sorting can in some version of the method alternatively comprise the following two selective acknowledgement steps. A first selective acknowledgement step of comparing the selective acknowledgements that have not been transferred to a sender. A second selective acknowledgement step of removing the redundant selective acknowledgements, i.e. removing selective acknowledgements only comprising redundant blocks that are included in other selective acknowledgements.
In some versions of the method the step of sorting is performed only if the previous selective acknowledgement has not been removed from the queue.
Preferably the method is performed on the output queue in any suitable layer from the TCP layer to the link layer, specifically the method is performed most advantageously on the internet protocol layer output queue.
By providing a method for removing Selective Acknowledgements (SACKS) from an output queue a plurality of advantages over prior art systems are obtained. A primary purpose of the invention is to, in a simple manner, enable the use of highly asymmetrical networks. According to the invention this is achieved primarily by making simple, and thus quickly executable, comparisons between the contents of new SACKs and previous SACKs and removing a previous SACK if the content is redundant and/or invalid. There is thus no loss of SACK information which could lead to unnecessary retransmissions. By reducing the number of SACKs that have to be sent back, a network with a higher asymmetry factor can be used. Other advantages of this invention will become apparent from the description.